The present invention relates to a temperature control network for a toner fusing device in an electrostatic copier, which has a heating unit such as an infrared lamp and a transporting means such as a conveyor, roller and the like, and is adapted to heat and fuse a copy image made visible by means of a fusible resin toner.
In the toner fusing device, it is imperative to maintain the temperature in the toner fusing device at a given temperature, hereinafter referred to as the toner fusing temperature, in order to produce a uniform fusing of a copy image and to prevent the toner fusing device from overheating. To this end, temperature control means such as disclosed in U.S. Pat. No. 3,553,429 or U.S. Pat. No. 3,558,853 and the like, have been proposed, in which there is provided a thermistor to control the electric power to be supplied to a heating unit. However, the electrostatic copier having the aforesaid fusing device suffers from disadvantages in that the toner image remains unfused, until the toner fusing device at the start of operation reaches a toner fusible temperature, hereinafter referred to as the toner fusible temperature, so that the copying operation should be suspended during heat-up and on the termination of copying, when all functions of the copier have been stopped, thereby the heated air residing within the fusing device effects an adverse influence on the other devices in the copier, such as the photosensitive member and the like, so that such heated air should be rapidly discharged to the exterior of the copier, until the toner fusing device reaches a temperature which would not adversely effect the other devices, such temperature being hereinafter referred to as a safe rest temperature. As a result, it is desirable that the temperature control means for use in a toner fusing device in the electrostatic copier be provided with the above functions. However, since the aforesaid toner fusing temperature, toner fusible temperature and safe rest temperatures are different relative to each other, the prior art copier should use a plurality of temperature sensing members for separately detecting three temperatures to control the other devices in the copier. This leads to the use of a complicated circuit in the temperature control means in the prior art electrostatic copier, with the accompanying need to provide a plurality of temperature sensing members in the toner fusing device.